Do you host all your sites on one server?

I host most sites on one dedicated server. I have spare servers in other data centers though (application servers and closed extranet servers) which I can quickly load temporarily with my main websites in an emergency situation when the main webserver goes down.

My experience is that one dedicated server has less problems with downtime than a number of shared hosting accounts. But you need to think of a restore and disaster recovery plan in advance, because if something goes wrong with your server, all your sites will be down at once.

I have sites on my main server which are interlinked and are sharing the same IP address. I have no indication that this has caused problems in the search engines. Hosting on different locations to hide interlinking between own sites may not be the right approach nowadays anyway because at least Google is also a registrar and has direct access to the database with the owner info of all domains, and can therefore easily compute which sites belong to the same owner, even if they are on different subnets or in different data centers.

We have a couple dedicated servers we use. One is kind of a sandbox and runs our smaller sites. The other is our production box and runs all the money makers.

Completely agree about a disaster recovery plan. Have some offsite backup service. And actually practice a restore!

Another consideration is managing the server. Many hosting companies leave that up to the owner. Some offer management services for a fee. If you are not comfortable managing the security and patches on your server, you may want to find a host that will do it for you.

If uptime is critical consider using different companies for hosting and DNS management. Backups are useless if the server is down and you can't point your DNS to somewhere else in a jam.

I have my websites on a VPS and a shared server, both at the same provider. I also have a different DNS provider who is not my web server company.

Lately I've been thinking that I should keep my VPS with the current company but also get a shared account at another place and move the shared sites over there.

I have had times when either server is down but I have not yet had a case where the whole web provider was unreachable.

I have spare servers in other data centers though (application servers and closed extranet servers) which I can quickly load temporarily with my main websites in an emergency situation when the main webserver goes down

What about DNS? I know it takes awhile for changes to propagate all over the net.

Another consideration is managing the server

Good point. I know nothing about running a server, and a managed dedicated plan can get quite expensive.

If uptime is critical..

Not more critical then other websites I guess. Critical for me as I hate it when my websites are not reacheable. Don't get me wrong, my current hosts provide good uptime (~99.95%). I just want to try a dedicated solution in an attempt to provide faster loading pages for my visitors and who knows maybe my websites will start growing at a faster rate.

I use a third party DNS provider where changes on the zone file are visible to the outside world are visible immediately. Users from new places will therefore see directly the site at the new IP address. Users who visited recently or who are behind a caching DNS server may have to wait until the TTL is expired and a new DNS request is sent.

I'm too host my sites on a dedicated server, but I have one rule - no more 20 sites on one server.

Nope. I have only my ecommerce site on the dedicated server. Don't need hackers snooping around my hobby sites looking for holes. It's easier to keep track of the most important site by itself.

I love dedicated servers too. If you go dedicated now go for a 'cloud' dedicated. That way you can back it up automatically or with a click.

Disclaimer: I work for a web host that sells cloud servers, but there are several so pick the one that sounds good to you.